Prior art reaction injection molding (RIM) process involves high pressure impingement of multi-component materials, such as urethanes, epoxys, silicone, polyesters, and phenolics, in a liquid form which react within the mold to form solid plastic products. In order to strengthen such products chopped glass fibers may be introduced into one or both reactant liquid components which tend to produce inherent serious wear problems in any valving which involves a high pressure drop over restricted passages in opening or closing a valve port or in metering high velocity flow with abrasion of glass fibers on restricted valve surfaces. Also pressurized flow through restricted valve ports causes heating of the materials with attendant changes in viscosity, flow rates and accuracy of injection charge, both as to quantity and ratio, required for optimum molded product results.
In a prior art pressure developing circuit which did not involve the use of chopped glass fiber reinforcement, a general recirculating system is pressurized by the closing of bypass valves following which flow produced by hydraulic metering cylinders establish high mixing head pressures against pressure developing balance valves in parallel with the closed bypass valves at which time the chemicals flowing through the pressure balance valves generate adverse high temperature effects. In order to partially alleviate such heating and shorten the time for pressure buildup "dead head" valves have been installed just after the mixing heads which block flow from metering cylinders delivering material to the system and cause required injection pressures to build quickly in the system. When line pressure in each half of the total system reaches the correct level, its dead head valve opens with high pressure maintained by its restricted pressure balance valve for a brief time delay after both dead head valves have opened. Mixing head valves are then opened simultaneously to start the timed mixing cycle during which flow is completely diverted through the mixing heads. Flow through the pressure balance valves during dead head buildup to operating pressure is thereby avoided and time and heating effects are reduced. However, temporary flow through the pressure balance valves after dead head opening and before mixing continues to provide undesired material heating effects; also, such valves continue subject to erosion problems incident to chopped glass fibers.